A low level concentration of a heavy metal, in the ppm range, is often difficult to remove from an aqueous chemical waste effluent. For example, waste effluent from uranium processing may contain about 1-150 ppm uranium. Various methods are available to reduce uranium levels to about 150 ppm by using, for example, a precipitation or solvent extraction process. However, the lower the level of uranium, the more difficult it is to remove. This is particularly true when removal is desired to sub-ppm levels, for example to a level of equal to or less than 0.1 ppm.
Aqueous waste in particular is difficult to treat to remove the heavy metal such as uranium to sub ppm levels as such a waste stream typically contains other waste components such as sulfates, nitrates, fluorides, chlorides and carbonates, which act as complexing anions and form strong complexes with heavy metals. These anions may be present in amounts of up to or over 20 g/l fluoride, 75 g/l sulfate, 50 g/l nitrate, 200 g/l chloride, 1200 ppm total carbonate. This makes the removal of a heavy metal to sub-ppm levels nearly impossible to achieve by conventional processes such as precipitation, solvent extraction or ion exchange. There are no known commercial processes which will reduce the concentration of a heavy metal such as uranium to sub-ppm levels on a plant process scale from such solutions. Even conventional strong acid or weak acid cation resins are not effective if complexing anions are present above a few hundred ppm in concentration.
One method currently under consideration is the use of a concentrated sodium silicate precipitation process. This involves adding concentrated sodium silicate to the waste stream, precipitation of the sodium silicate to scavenge the heavy metal from the waste stream and filtration to remove the precipitate which is either treated to release the heavy metal for recycle or disposed of. Such a process is complex and requires expensive solids handling steps.